1. Field of the Invention
The present invention relates to a monitoring system for optical regenerators, and more particularly for all optical regenerators having such functions as reshaping, regeneration and retiming.
2. Description of Related Art
Optical repeaters are used in long distance optical repeating transmission systems including optical submarine cable systems. In an optical repeating transmission system in which an optical terminal equipment and an optical repeater are connected to each other by an optical transmission fiber, it is of critical importance to the construction and maintenance of the system to monitor the operating state of the installed optical repeater.
Especially in an optical submarine cable system, once an optical repeater is installed, if the repeater runs into trouble, it will cost enormous time and money to retrieve and repair the faulty repeater. Therefore, even the slightest trouble is impermissible for the repeater, which has to be reliable enough to remain usable for a long period after installation without needing maintenance work. Furthermore, it is essential to monitor the whole system so that, should any trouble arise, switching over to a standby unit could be immediately achieved.
According to the prior art, optical amplifiers, which directly amplify transmitted optical signals as they are without converting them into electrical signals, are used as the aforementioned optical repeaters. Optical amplifiers used as repeaters are known as optical amplifying repeaters. A conventional optical amplifying repeater monitors the state of each part within itself with monitor signals. Such an optical amplifying repeater is provided with an electric converter for converting each monitor signal into a voltage corresponding to its content, a frequency converter for converting this voltage into an electric signal having a repetition frequency corresponding to the voltage, and an amplitude controller for controlling the amplitude of the electric signal having gone through this frequency conversion. Such a conventional optical amplifying repeater also has a pumping source whose driving current is amplitude-modulated by monitor signals, which are amplitude-controlled by the amplitude controller, and an optical coupler for leading the output of this pumping source to an optical amplification fiber.
As optical amplification fiber, an optical fiber doped with erbium, which is a rare earth element, is commonly used. As signal light, a ray of 1.55 .mu.m in wavelength is used, and as excitation light, one of 1.48 .mu.m in wavelength is employed. As optical coupler, an optical wavelength division multiplexer is used with a view to efficient coupling of the signal light and the excitation light.
An optical signal, when entered in an excited state resulting from the input of excitation light to the erbium-doped fiber, is amplified with a certain gain and outputted. In a conventional optical repeater, said monitor signal is superposed over the main signal by the amplitude modulation of this gain itself, and delivered outside.
Incidentally, all optical regenerators are now attracting note as optical repeaters for the optical repeating transmission systems of the next generation. Characteristically, they can reshape, amplify, regenerate and retime optical signals as they are. Repeaters having any two out of the three functions of reshaping, amplification and regeneration are known as optical 2R regenerators. Similarly, repeaters having all of said three functions are known as optical 3R regenerators. Known studies on regeneration include one contained in OFC (Optical Fiber Communication) '95 Conference Edition Technical Digest, Volume 8, WH4, "Noise compression and waveform restoration by an optical signal regenerator."
Optical repeating transmission systems having these functions involve the problem of inapplicability to conventional monitoring systems because they regenerate optical systems. Namely, in such conventional monitoring systems, monitor signals are superposed over the main signals by amplitude modulation. Therefore, when the main signals are regenerated, the amplitude modulation is removed and the monitor signals superposed over the main signals are lost. Accordingly, there is needed a monitoring system which is applicable to the aforementioned all optical regenerators as well.